Oxymethylene polymers, as that term is used herein and further defined below, means those polymers having recurring --OCH.sub.2 --units directly attached to each other. Such polymers have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde. Oxymethylene copolymers have at least one chain containing recurring oxymethylene units interspersed with --OR--groups in the main polymer chain, where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the polymer chain between the two valences, with any substituents on the R radical being inert, that is those which are free of interfering functional groups and will not induce undesirable reactions. Particularly preferred are copolymers which contain from 60 to 99.9 mol percent of recurring oxymethylene groups. The R may be, for example, an alkylene or substituted alkylene group containing at least two carbon atoms.
Among the copolymers which may be utilized are those having a structure comprising recurring units having the formula: ##STR1## wherein n is an integer from zero to 5 and wherein n is zero in from 60 to 99.9 percent of the recurring units. R.sub.1 and R.sub.2 are inert substituents, that is, substituents which are free of interferring functional groups and will not induce undesirable reactions.
Particularly preferred oxymethylene copolymers are those having incorporated therein oxyalkylene units having adjacent carbon atoms which are derived from cyclic ethers having adjacent carbon atoms. These copolymers may be prepared by copolymerizing trioxane with a cyclic ether having the structure: ##STR2## wherein n is an integer from zero to 2.
Examples of preferred polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms such as the copolymers disclosed in U.S. Pat. No. 3,027,352, incorporated herein by reference. Among the specific cyclic ethers which may be used are ethylene oxide; 1,3-dioxolane; 1,3,5-trioxepane; 1,3-dioxane; trimethylene oxide; pentamethylene oxide; 1,2-propylene oxide; 1,2-butylene oxide; neopentyl glycol formal; pentaerythritol diformal; paraldehyde; tetrahydrofuran and butadiene monoxide. As used in the specification and claims of this application, the term "copolymer" means polymers having two or more monomeric groups, including terpolymers and higher polymers.
After polymerization, oxymethylene polymers, such as those comprising trioxane-ethylene oxide copolymer chains, contain unstable polyformaldehyde ends, which must be removed in order to improve the thermal stability and other properties of the acetal copolymer. For this purpose, a hydrolysis process, such as that disclosed in U.S. Pat. No. 3,219,623, incorporated herein by reference, may be used. More specifically, a melt hydrolysis process, such as that disclosed in U.S. Pat. Nos. 3,318,848 and 3,418,280, incorporated herein by reference, is preferably utilized.
In the past, it was very difficult to remove enough volatile material (e.g. trioxane, formaldehyde, formic acid, water, hydrolysis agent, etc.) to provide a product suitable for direct use. Subsequent devolatilization and compounding were required to remove volatiles in separate steps, to avoid excessive color formation.
The presence of too high a level of formaldehyde leads to a material which can easily form mold deposits. These mold deposits may be of a paraformaldehyde type that result from residual formaldehyde in the polymer. The presence of a significant amount of formaldehyde can also lead to yellowing of the polymer by a number of possible mechanisms. Aldol condensation of formaldehyde, followed by dehydration, forms resin with increased color. This can necessitate frequent corrective action in a liquid phase polymer process. When the oxymethylene polymer is in slurry form, polymer color is highly affected by iron and other cations. Hydrolysis agents such as triethylamine, which can be used to control pH, readily increase color when the concentrations of formaldehyde and hydrolysis agent exceed the recommended limits during processing.
Color in oxymethylene polymers may also be induced by oxidation, which can result from exposure to air or from high formaldehyde content. This would indicate that oxidation reactions either produce chromophors (radicals which can lead to color formation) or that the reactions produce formaldehyde, which can generate chromophors. In processes for producing oxymethylene polymers, it is also known that color is induced by high process temperatures
as well as by high formaldehyde content. Because the presence of heat and formaldehyde provides a product which has a high potential for the formation of color, removing formaldehyde and minimizing the heat history of the polymer will minimize color formation in oxymethylene polymers.
Accordingly, there exists a need in the art for a method of devolatilizing oxymethylene polymers to remove formaldehyde and other volatile materials, such as triethylamine, water and catalyst residues, thereby reducing the formation of mold deposits, minimizing color formation and providing a more stable oxymethylene polymer which can be readily extruded or molded into shaped articles. The process of the present invention provides an answer to that need and also allows the addition of stabilizers without the necessity for remelting and substantially avoids the formation of excessive color.